An electronic device utilizing a GaAs III-V compound semiconductor has actively applied to an ultra-high speed transistor, taking advantage of features such as an ability to operate at an ultra-high speed and at a higher frequency because of its high electron mobility, and has recently been used practically as various essential components of high-frequency communication instruments such as a cell phone because of its advantage of low power consumption.
As the above described ultra-high speed transistor, a high electron mobility field effect transistor (referred to as a HEMT, hereinafter) has been well known. The HEMT is also referred to as a high electron mobility transistor, a modulation doped field effect transistor (MODFET), or a hetero-junction field effect transistor (HJFET).
The HEMT is principally characterized by adopting a selective doped hetero structure which is composed of an electron supplying layer for supplying electrons (a doped layer) and a channel layer through which electrons run, these layers being made of different materials. In this hetero structure, electrons supplied from n-type impurities within the electron supplying layer are pooled in a potential well formed at a channel side of a hetero-junction interface due to a difference of electron affinity between materials constituting the hetero junction to form a two-dimensional electron gas. Since n-type impurities supplying electrons are present in the electron supplying layer while electrons are present in a high purity channel as described above so that the ionized impurities and the electrons are spatially separated from each other, the two-dimensional electron gas within the channel is hardly scattered by the ionized impurities, and consequently a high electron mobility is realized.
The HEMT has usually been fabricated by using an epitaxial substrate in which thin film crystal layers respectively having predetermined electronic characteristics are laminated and grown on a GaAs single crystal substrate so as to have a predetermined structure. It is important for the HEMT to have a high electron mobility in the channel. Therefore, since it has been required to precisely control the thin film crystal layer forming the HEMT structure on the order of monoatomic layer level, a molecular beam epitaxy (referred to as a MBE method, hereinafter) or a metalorganic chemical vapor deposition (referred to as a MOCVD method, hereinafter) has conventionally been used as a method for manufacturing an epitaxial substrate having a HEMT structure.
It has been said that the MBE method, which is one of the vacuum evaporation methods, is excellent in the layer thickness and interface controllability, while this MBE method is inferior in the mass-productivity. On the contrary, the MOCVD method uses an organometallic compound or a hydride of atomic species constituting an epitaxial layer as a source material and then pyrolyzes the source material on the single crystal substrate to grow a crystal thereof, so that this method is applicable to a wide range of substances used as the source materials and has a feature of extremely extensively and precisely controlling the composition and thickness of the epitaxial crystal, and consequently this method is suitable for the purpose of processing a large amount of substrates with the favorable reproducibility.
In addition, recent and rapid technical innovation of the MOCVD method has allowed for not only controlling of the impurity amounts but also realizing a steep hetero interface or favorable in-plane uniformity that has never been considered as possible by this method. In fact, an epitaxial substrate fabricated by the MOCVD method is in no way inferior to that fabricated by the MBE method in terms of a characteristic such as an electron mobility of the HEMT, and has commercially and widely been used.
Thus, since the HEMT is an ultra-high speed transistor utilizing a two-dimensional electron gas having a high electron mobility, an electron mobility of the channel layer which is as high as possible is favorable for obtaining a high-performance HEMT. Therefore, InGaAs has recently been used as a material for the channel layer instead of using GaAs, because InGaAs is excellent in its electron transporting characteristic as well as being capable of significantly changing its energy gap in accordance with the In composition and further capable of effectively confining the two-dimensional electrons. In addition, AlGaAs or GaAs is known as a material to be combined with InGaAs.
InGaAs has a property such that the mobility becomes higher with the increase of the In composition. Thus the transistor can be sophisticated, however, a lattice constant of InGaAs also becomes larger with the increase of the In composition, which leads to lattice misfit with the electron supplying layer or the substrate material. For this reason, a method in which crystal growth is performed in a pseudomorphic condition has been used. This method utilizes a property in which a high quality crystal layer can be fabricated without inducing lattice disorder such as dislocation although the lattice strains and elastically deformed, provided that the grown layer thickness is below a certain layer thickness referred to as a critical thickness even if the crystal growth involves the lattice misfit between materials having different lattice constants. The HEMT, in which an InGaAs strain layer is used as a channel layer, is referred to as a pseudomorphic high electron mobility field effect transistor (hereinafter, referred to as a pHEMT).
It has been known that a critical thickness of the InGaAs layer is given as a function of the In composition and the layer thickness. As for an InGaAs layer with respect to a GaAs layer for example, the critical thickness is expressed by a theoretical equation disclosed in J. Crystal Growth, 27 (1974) p. 118 and in J. Crystal Growth, 32 (1974) p. 265, and this theoretical equation has been found to be experimentally correct as a whole. In addition, JP-A-6-21106 discloses an epitaxial substrate which allows for efficient manufacture of a pHEMT having a high mobility, the epitaxial substrate having an In composition within a certain range which is further limited by using a certain relational expression based on a certain relation between an In composition and a layer thickness defined by this theoretical equation. Indeed, an InGaAs layer having an In composition of 0.20 and a layer thickness of about 13 nm has been practically used as an InGaAs strain channel layer which can be epitaxially grown without inducing a reduction in its crystallinity.
Further, it is effective to additionally reduce the scattering of two-dimensional electrons caused by ionized impurities in order to improve the mobility. Thus, a spacer layer which has the same material and composition as an electron supplying layer but to which any impurities are not added may be inserted between the electron supplying layer and the channel layer. For example, Japanese Patent No. 2708863 discloses a structure for improving a two-dimensional electron gas concentration and an electron mobility, in which a spacer layer consisting of an AlGaAs layer and a GaAs layer is inserted between an InGaAs strain layer used as a channel layer and an n-AlGaAs electron supplying layer of a pHEMT structure in order to optimize the growth conditions.
A pHEMT which uses a strain InGaAs layer as a channel layer as described above can confine a large number of two-dimensional electrons under the effective influence of quantum effect by making an In composition of the channel layer larger and making a band gap between the channel layer and the electron supplying layer or spacer layer wider, so that the pHEMT has an advantage that the improvement of electron mobility can be compatible with the high mobility.
It is well known that the electron mobility is an important parameter for improving various characteristics such as an on-resistance, a maximum current value, or a transconductance each of which is an important performance indicator of a field effect transistor. Therefore, further improvement of the electron mobility can achieve a reduction of build-up resistance (on-resistance), which leads to a reduction of power consumption. In addition, since a calorific value can be decreased by lowering the power consumption, it is possible not only to realize higher integration of a device but also to reduce the chip size, so that the number of chips manufactured from one epitaxial substrate can be increased and a degree of freedom of modular design can also be increased. From this viewpoint, it is desired to further improve the electron mobility in the case of a pHEMT which is used for various portable instruments such as a cell phone.
However, the electron mobility in the epitaxial substrate having the pHEMT structure has not yet reached a satisfactory level, in view of possibility that the characteristics of the transistor can be further improved by increasing the two-dimensional electron gas concentration and the electron mobility simultaneously. For example, as described in “Compound Semiconductor, Materials Head into Mass Production Stages (2)”—Compound Semiconductor Materials for Electronic Devices—, Yohei Otoki and in Semiconductor Industry News Forum “All about Compound Semiconductors 2002—Movement of Optical High Frequency Devices toward Reemerge”, Jun. 5, 2002, Myoujin Kaikan, Ochanomizu, Tokyo, a maximum value of the electron mobility in the channel layer at room temperature (300K) which had been reported with respect to the pHEMT structure epitaxial substrate was about 8170 cm2/V·s at a two-dimensional electron concentration of 2.06×1012/cm2, and about 7970 cm2/V·s at a two-dimensional electron concentration of 2.77×1012/cm2.
The electron mobility in the two-dimensional electron gas at room temperature (300K) has been considered to be determined by the scattering due to a crystal lattice and the effective mass of the GaAs electrons. Thus, if a strain InGaAs layer is used as a channel layer to which In is added, it is expected that the effective mass of electrons decreases and the electron mobility increases, but on the contrary, there is concern that an increase in alloy scattering due to In and Ga may result in a reduction in the electron mobility. Further, the effective mass of electrons has been considered to develop anisotropy along a vertical direction and a horizontal direction with respect to a two-dimensional electron gas plane, and there has not yet been reported an effective mass along the horizontal direction which is practically important. Thus, under the present circumstances, any measures have not been taken which ensure a reduction of the effective mass of electrons and the improvement of electron mobility.
On the other hand, it has already been reported that InGaAs which has lattice-matched with an InP single crystal substrate to be used has an electron mobility of 10000 cm2/V·s at room temperature. In addition, the so-called metamorphic technology has recently been developed for forming a buffer layer whose lattice constant is near a lattice constant of InP by changing a lattice constant of the buffer layer to a lattice constant of InP in stages, provided that an InAlAs is used as a buffer layer on the GaAs single crystal substrate and an In mixed crystal ratio is changed in stages. An electron mobility exceeding 9000 cm2/V·s has been reported by forming a modulation doped structure having an InGaAs layer as a channel on the buffer layer formed by using this technology. That is, InGaAs without strain channel allows for a higher electron mobility which exceeds GaAs and is consistent with a low electron effective mass thereof.
When a high-performance transistor having a high electron mobility such as exceeding 8200 cm2/V·s is achieved as described above, it is necessary to use an InP single crystal substrate or to laminate a special buffer layer on a GaAs single crystal substrate in accordance with the above described metamorphic technology. However, if the InP single crystal substrate is used, it is necessary to use an epitaxial substrate having a modulation doped structure comprising as a channel layer an InGaAs layer being lattice-matched with InP and as an electron supplying layer an InAlAs layer being lattice-matched with this channel layer. For this reason, source material cost becomes extremely high when an InP single crystal substrate is used. On the other hand, even the metamorphic technology to be used, in which a thick buffer layer is formed, has a problem that the production cost becomes larger and also a new material processing technology which is different from that in the case of conventional GaAs material is required, and further has a problem that a low reliability is provided due to a high crystal defect density in the buffer layer.
Thus, in the pHEMT structure epitaxial substrate which uses an n-AlGaAs layer as an electron supplying layer and a strain InGaAs layer as a channel layer, a further improved epitaxial substrate is strongly desired which has a higher two-dimensional electron gas concentration together with a higher electron mobility compared with the currently reported values.